Ingot molds



E. MARBURG 2,734,242

INGOT MOLDS Feb. 14, 1956 Filed June 30, 1954 v4 SheetS-Sheet l Feb. 14,1956 E. MARBURG 2,734,242

INGOT Moms Filed June 30, 1954 4 Sheets-Sheet 2 77M HFT/5e Pou/e M//Varf0075/05 60e/9064 Wan/5 /A/5/5 @Bevagna/V5 /N/r/.qL I

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wm@ i Feb. 14, 1956 E. MARBURG 2,734,242

INGOT MOLDS Filed June 30, 1954 4 Sheets-Shaml 4 United States Patent OINGoT MoLDs dgar Marburg, Pittsburgh, Pa., assignor to United StatesSteel Corporation, a corporation of New .iersey Applicaties .time 30,1954, serial N0. 440,353

4 Claims. (Cl. 22-139) This invention relates to improvements in ingotmolds.

The present application is a continuation-impart of my earlierdepending, application Serial No. 299,635, led July 18,. 1952,v nowabandoned.

Both the uniformity of composition and the internal soundness of a steelingot. are dependent upon its mode of solidilication. In changing fromthe liquid to the solid state, steel contracts about 4 percent; incooling from the solid'us temperature (at which solidication is justcornplete) to atmospheric temperature, steel contracts an additionalfl'percent. As4 a result oi?l contraction during solidicati'on and cooling,a central cavity or pipe, which may extend below midhegh`t, develops iningots cast without top protection. The continuitj of the pipe may bebroken by bridging; that is, complete trans-'solidication, at one orseveral places below the top of the ingot. Pipe cavity above theuppermost bridge is called open or primary pipe; that below this bridgeis referred to as' secondary pipe. To prevent pipe from developing iningots, collars of refractory insulating material (generally tire' clay)are set on` mold tops; they extend a'few inches down inside the mold.The hot top is poured after the mold is filled. Since ,the metal inthehot top, or sinkhead, remainsliquid longer than that in the ingot body,it may drain in'to the latter and compensate for ingot shrinkage. Forthe' purpose of describing and claiming the present invention, I definethe term ingot body as refer-ring to the portion of an ingot below thehot-top junction (plane of lower edge. of hot top) in a hottopped ingot,and the entire ingot below the height of pour in a non-hot-topped ingot.

Initial' soliditi'cation of ingots occurs approximately according toFeilds Equation d=k\/ (d=depth in inches; t=tirne in minutes). If a' beplotted against t in this equation, a parabolic curve results. Accordingto this curve, rates of solidiication are extremely fast. near thesurface of an ingot body; they decrease to slow rates in the interior.

Steel solidies as dendrites, or tree-like crystals. Since purer metalsolidifies at a higher temperature than that at which less pure metalsolidies, the solidifying crystals are always purer than'. the adjacentliquid metal. In particular the layer of' liquid metalA immediatelyadjacent tol the face of solidieation has a higher content ofsegregating' efe'ments, principally carbon, phosphorus and sulphur, thandoes either the solid metal or the remaining liquid metal. Thel degreeof segregation. that occurs is a function of the rate of heatextraction. Nearthe surface of an, ingot body, where heat extraction andsolidification rates are extremely fast, little or no segregationoccurs; as solidiication rates decrease inwardly, the intensity ofsegregation in the segregated layer increases.

The segregated liquid layer may be entrapped by the following mechanism:elongated or columnar crystals `grow' inwardly approximatelyperpendicular to the surface of an ingot body. A columnar dendrite maygrow slightly in advance of its neighbors,` and penetrate the segregatedliquid layer. Since the segregated layer is slightly in- 'ice sulating,penetrating columnar crystals provide a preferred course for heat flow.Thus solidiiication may proceed inwardly and upwardly beyond thesegregated layer, and entrap the latter. Because transverse solidicationproceeds inwardly at the same time, the above mechanism results inlayers of segregation that slope inwardly toward the top of the ingotbody, known as inverted-V segregation. ln large ingots the abovemechanism is repeated, and a series of approximately parallel lines ofinverted-V segregation isv developed.

Another type of segregation in ingots is V segregation, which occursalong the vertical axis. V segregation results from the entrapment ofsegregated liquid layers adjacent to the face of vertical solidicationby the soliditication at higher temperature of purer metal above thesegregated layers. V segregation is most concentrated in the upperportions of ingot bodies and decreases in intensity downwardly. Inbig-end-up ingots V segregation is generally confined to the upper 15percent of the ingot body; it may extend below mid-height in big-enddowningots. v

if liquid metal be completely entrapped by solid metal, pores or voidsresult from contraction of theA entrapped metal in solidifying. Thusporosity of aY minor degree is associated with both and. inverted-Vsegregation. Major porosity develops along the vertical axes of ingotsof certain: steels asV a result of the entrapment of a vertical columnot' incompletely solidified metal by transso]iditicationy justy below.the hot-top junction. This defeet, referred to as axial porosity, issomewhat-,similar ink appearance to V segregation. But, whereas thelatter is primarily segregationwith associated porosity, the former is Vporosity with associated segregation. n Y

The axial defects described above, V segregation and V porosity, are themost serious internal defects in ingot bodies, and are objectionable formany product applications. Because ot high breakage in forming sheet andstrip products from the upper portions of ingot bodies, the top cuts ofingot bodies of deep-drawing steels are often diverted to other use.Forging may cause internal cracks to develop in ingot bodies withunsound centers; axial defects may also cause weakness in forgedproducts. or adversely atect their surface appearance. In prodnets thatareV center-drilled or bored, center segregation may adversely affecttool life, and/or maycause defects in surface of bore. Laminations insheet or strip .product, blisters in galvanized sheet or strip product,and pinholes in tin plate are other product defects caused by unsoundingot centers.

An object of the present invention is to provide irnprov'ed ingot moldswhich minimize axial segregation and porosity in ingot bodies casttherein.

As hereinafter explained', it is desirable that ingot bodies solidify'as rapidly as possible in a vertical direction upwardly from the bottom,and as slowly as possible transversely across the top; a further objectis to provide ingot molds which promote vertical solidiiication, andretard transverse solidiiication across the top.

A further object is to provide ingot molds in which the foregoing.objects are attained by simple proportioning of the thickness of variousparts of the mold walls.

In accomplishing. these and. other objectsy of the invention, i haveprovidedv improved details of structure, preferred forms of which areshown in the accompanying drawings, inwhich: v

Figure l is a schematic view which shows a typical soliditicationpattern of a big-end-up ingot;

Figure 2 is a schematic View which shows a typical soliditicationpattern of a big-end-down ingot;

Figure 3 is a top plan view of a big-end-up ingot mold which embodiesfeaturesv ofthe present invention;

Figure 4 is a side elevational view of the mold shown in Figure 3, themold being equipped with a hot top;

Figure 5 is a horizontal sectional view taken on line V-V of Figure 4;

Figure 6 is a horizontal sectional view taken on line VI-VI of Figure 4;

Figure 7 is a horizontal sectional view taken on line VII-VII of Figure4;

Figure 8 is a vertical sectional View of a big-end-down mold constructedin accordance with the invention, the mold being equipped with a hottop;

Figure 9 is a view similar to Figure 8, except that the mold lacks a hottop;

Figure l0 is a vertical sectional view of a modified form of abig-end-down mold in accordance with my invention, the mold beingequipped with a hot top; and

Figure ll is a graph which compares mold wall temperatures of a moldconstructed as shown in Figure 8 with those of a conventional mold.

The progress of soliditication of an ingot body may be depicted byisochrones representing the face of solidilication at various intervalsafter pour. Figure l shows the solidiication pattern of a 32 by 32 inchbig-end-up ingot reconstructed from measurements of shell thicknesses ofsix ingots dumped at various intervals after pour. Figure 2 shows asolidication pattern for the 29-inch-wide section of a-29 by 66 inchbig-end-down ingot. Although the patterns for the big-end-down and thebig-end-up ingot are generally similar, they differ in certain importantrespects, to be discussed below. The patterns may be considered fromthree viewpoints: (a) solidiiication transversely from sides to middle;(b) solidication vertically from base to top; and (c) acceleratedsolidiication resulting from overlapping of transverse and verticalcomponents of heat extraction, originating at points of tangency FeildsEquation d=k\/t. I have found that inner lines of inverted-V segregationdevelop as a result of acceleration of transverse solidification; hencethey coincide with loci af of this acceleration. Accelerated `transversesolidiication is first completed to the middle of the ingot at point c,which represents the tip of the base cone abaca of verticalsoliditication. When trans-soliditication is completed at c, thereremains a relatively large column of liquid metal above point c, asbounded in Figure l approximately by isochrone 100. At point ctransverse components of heat extraction from all four sides of theingot are added to vertical components effecting verticalsoliditication. As a result, vertical solidification rates are greaty lyaccelerated at point c, as indicated by the wide spacing of isochronesalong the vertical axis above this point. Because of the extremely fastrates of final accelerated vertical solidification above point c,vertical solidication reaches the top of the ingot body beforetransverse solidification is completed across the top. Dashed lines ecetrace the cores of vertical soliditication thus developed in ingotbodies of both types.

I have found that V segregation is developed within and extends acrossthe vertical cores of ingot bodies. Since segregates are lighter thanliquid steel, they have a tendency to float upwardly and escape into thesinkhead. Their ability to do so, however, depends upon the width of thevertical core and that of the adjacent zones of acceleratedtransverse'soliditication at the hot-top junction, that is, upon widthsVt and Vu. As indicated by the s0- lidication patterns, Vt and Va arewider in big-end-up ingots than in big-end-down ingots of comparablesizes. For this reason, the former ingot bodies develop less axialsegregation and porosity than do the latter.

Widths Vt and Va are principally dependent upon two factors;` (l) thewidth to height (w/h) ratio of an ingot body, and (2) the ratio of therates of transverse solidification across the top of the ingot body tothose of vertical soliditication. Wide vertical cores and sound ingotbodies are favored by high w/h ratios. Practical limitations of rollingmill equipment and economic considerations, however, dictate the upperlimit of w/h ratios of ingot bodies. It would almost certainly proveuneconomical to increase w/h ratios of ingot bodies to the extentnecessary to eliminate all axial defects.

Wide vertical cores (and sound ingot bodies) are also favored by slowrates of transverse soliditication across the top of an ingot relativeto vertical solidication. Since experiments have demonstrated thattransverse solidification rates vary with mold-wall thickness (at leastin mold walls over 2 inches thick)it would naturally seem desirable todesign mold walls thin at the top, and thick at the base (since rates ofaccelerated vertical solidilication in the vertical core are causedprincipally by transverse components of heat extraction to the lowerportion of the mold wall). Indeed, mold walls are commonly so designed,with the base 1 to 4 inches thicker than the top, and with a uniformtaper between the two ends. Unfortunately, however, tapered mold wallsare ineffective in producing differential solidication in ingots.Numerous dumped-ingot experiments have revealed that, except foraccelerated soliditication from the base, solidilication proceeds atuniform rates throughout the height of ingots cast in molds with taperedwalls. It is extremely diicult to analyze exactly heat losses from moldwalls. Heat will ow through the walls, vertically as well astransversely, from hotter to colder portions. In addition to conductedheat, heat will be radiated from the outer surface in amounts varyingwith the surface temperature. Regardless of the exact analysis of heatlosses from mold walls, however, it is well-known that molds becomehotter in their middle portions than at their end portions (this can beverified by observation; an oval-shaped portion in the middle of eachwall becomes heated to a bright red to orange color, while the endportions remain dull red to black). Since heat-extraction rates willvary inversely with mold temperatures, however, it is undesirable t0have a higher temperature at the middle than at the top of the moldwall.

The purpose of the present mold design, as will be clear from thefollowing description, is to control temperatures in mold walls so as tocause greater rates of transverse heat-extraction from the base andmiddle than from the top portion of a mold, hence to causesolidification to proceed in a favorable manner.

Figures 3 to 7 show a big-end-upV mold constructed in accordance with myinvention. This mold is equipped with a hot top I-I which is of anystandard or desired construction and extends a short distance into thetop of the mold cavity. The upper portions 10 of the mold walls are thinand the lower portions 12 thick. Immediately below the bottom of thethin upper portion 10 is a relatively short, thickened section 13.Between this thickened section and the top of the thick bottom portion12 is a relatively short thin section 14. The trace of the insidesurface of the Walls in any vertical plane above the base is a straightline.

When molten steel is introduced to the mold just dcscribed, the thinportions 10 and 14 become hotter than the thick portions 12 and 13.Consequently heat ows from the top portion 10 and from the thin section14 into the thick section 13, as indicated by arrows. Hence a horizontalplane may be passed through section 13 across which no vertical heattiow occurs. Similarly heat flows upwardly from thin section 14 intothick section 13 and downwardly from thin section 14ito thick portion12. Hence a horizontal plane across which no vertical heat ow occurs mayalso be passed through thin section 14.

Because of the middle thick-and-thin arrangement, heat wenn# from theupper thinportion@ 111A tlows' downwardly into the comparatively smallvolume represented by the up* per half of the thick section: 13.Similarly the lower thick. portion: 12.re'ceivesheat only fromthezlowcrhalf of -thin section 14. Thus the. normally faster rates ofsolidiiication of thick base portion 12 and normally. slower rates: ofthin upper portion are dist-urbedtof at minimum) extent. Asa result, thethin` top and thick basepon tions ofzrnolds` are enabled toeffectdifferential soliditication yin ingotsin the manner desired.,

Figure 8 shows a big-endfdowrfr.I mold constructed in accordance with myinvention. This moldisl eq'tripped;A with a hot top H similar to thatshown in Figure' 4. The mold walls havea relatively thick lower portion17 and a relatively thin' upper portion 181 At the' bottom of the latteris a relatively narrow thick section I9. Between theA thick section 19and the top` of the thick lower portion 1"]"is` a relatively narrow thinsection` 20. The trac of' the inside surface in' any vertical planeagain is a straight line. Thick,section' 19 andj thin section 20 act tovdisrupt vertical ow of heat in the same manner as the correspondingthick section 13` andi thin section 14 the big-ed'up mold' shown in'Figures 3 to 7:.

Figure 9 shows a big-enddown mold which is similar to Figure 8, exceptthat it lacks a hot top. Semi-killed, or gas-evolving steels, such asrepresented in Figure 2, are commonly cast without hot tops. Moltenmetal is poured into the mold to a height a few inches short of the moldtop. The height of ingot body cast therein is comparable to the portionof a hot-topped ingot below the hot-top junction. The solidicationpattern of this ingot, Figure 2, has already been discussed.

Figure 10 shows an alternative mold construction which can be used tofacilitate manufacture. Although Figure 10 shows a big-end-down moldequipped with a hot top, it is apparent that the same mold can be usedwithout a hot top and that the same principles can be applied to abig-end-up mold. In large foundries, a sand slinger frequently is usedto make the outer sand mold for casting the ingot mold. A sand slingerhas revolving blades which throw sand with tremendous force into theperipheral space between an outer housing and the pattern (similar tothe ingot mold exterior). It is apparent that sand so placed would notpack tightly in the dead space below the thick ring 13 of Figure 4 or 19of Figures 8 and 9. Consequently the mold shown in Figure 10 has athickened portion 22 and a thinner portion 23 separated by a re-entrantangle modified to enable sand to be packed tightly with a slinger. Theaction of the thick and thin portions in the mold walls is the same asalready described for the other embodiments.

Height d in Figures 4, 8, 9 and l0 represents the vertical distancebetween the top of the ingot body and the beginning of the uppermostthickened section and may be evaluated as about 10 to 70 percent of theheight of the ingot body. While these are extreme limits ofeffectiveness, the preferred range is about 35 to 50 percent of theheight of the body. The actual mold extends a few inches above thisheight to allow for insertion of a hot top or to leave some clearanceabove the height of pour in a uonhottopped ingot. The minimum spacing ccan be as low as it is practicable to cast corrugations. The upper limitof spacing may be specified as about 24 inches. While these are extremelimitations, the preferred spacing of corrugations is about l2 inches.

Figure l1 shows a graph which demonstrates the electiveness of myinvention. In this graph the solid line curves represent mold walltemperatures at various vertical positions and at various intervalsafter pour with a mold constructed as shown in Figure 8, of insidedimensions 71/2 by 71/2 inches at the top, 81/2 by 81/2 inches at thebase. The broken line curves represent corresponding temperatures in aconventional mold (i. e. a mold whose walls taper uniformly), of thesame internal dimen sions, and of the same wall thickness at the top(l1/zvao increQLand at the base E .i1/zI inches) as the mold shown' inlrgnre it; In aan instance the. temperatures were' determined byinserting al thermocoupl in oneot'v the m'oldwallsl at thelreig'htindicatedand tol a depth 1,411 inch fromy the insidefface, a secondthern'iocouple in another wall at" thel same height' to themidfthickness depth,- and a third thermocouple in a different wall atthis height to a depth" lidi inch' from the outside face, and`awe'ragingy the three readings. 'rneb'rok-en-lne curves show that theconventional mold'v is hot-test a-tj the middle portion; also it ishotter at the baserA portion than vat theA top1 (at least for the aboutA7 minutes, required for most of this' ingot to'sol'idify). In arr-exactopposite manner, the. experiment-ali molo is least not at the middlethick rirrgfr the duration of solidiiie'ation-v it isr alsol less hot atthe base than atfthe top ponan. Thus the'experimentat mold. extractsheat faster front the'- base and middlepornon, than. it does from thetop of' an ingot (in the manner desired',v for favorablesolidiiication).

Another; test was conclut-:ted in which i'ng'ots of 0.25'- perc'enticarbonhseei cast in tl'lef above' two inol'ds 'were back=poured with.steel. containing 2 percent copper about 61 minnresan 30: secendsafterpour. Snce steel containing copper etches darker than steel withoutcopper, the metal,v that had been liquid at back-pour was revealed as adark column in the macroetch of a longitudinal section of each ingot.Measurements of these macroetches reveal that, at the time of back-pour,transverse soliditcation had proceeded 2%6 inches across the top of theingot cast in the regular mold, but only 2 inches across the top of theingot cast in the mold of special design, Figure 8, Verticalsolidilication had progressed to a height only about 51/2 inches abovethe base of the former mold; it had attained a height of 13 inches inthe latter mold. These macroetches provide evidence that verticalsolidification had progressed almost 21/2 times as far in the mold ofspecial design as in the regular mold; but transverse solidi` icationhad only proceeded 86 percent as far across the top of the former ingot.

Ingots of various grades,.plaincarbon, low-alloy, and

f sulfur steels, have been cast in big-end-up and big-enddown molds ofthe design shown in Figures 4 and 8. Comparison ingots of the samesteels have been cast in conventional molds of the same internaldimensions, but of uniform wall taper. The ingots have been splitlongitudinally along the center plane, machined and ground to a smoothsurface, and macroetched. All of the ingots cast in molds o-rconventional design contained secondary pipe, V segregation and axialporosity to a greater or less extent. In contrast, all the ingots castin the special molds with one exception (the big-end-down low-alloysteel ingot had some axial porosity) were completely solid and sound.

From the foregoing description it is seen that the present inventionaffords an effective means for disrupting the vertical llow of heat iningot mold walls by simple proportioning of the wall thickness. Theinvention effectively eliminates the most serious axial defects iningots, namely secondary pipe, V segregation and axial porosity.

While several embodiments of my invention have been shown and described,it will be apparent that other adaptations and modifications may be madewithout departing from the scope of the following claims.

I claim:

l. An ingot mold comprising solid side walls which define a cavityadapted to receive molten metal for casting an ingot body and whichextend a relatively short distance above the normal height of the ingotbody, said walls below this height being of greater thickness in theirlower portion than in their upper portion to accelerate solidilicationof the lower portion of an ingot body both transversely andverticallyand to retard transverse solidtication of theupper portion, the trace ofthe inside surfaces of said walls in any vertical plane away from thelower end being a straight line, and means on the exterior of said wallsbetween their thick and thin portions spaced below the normal height ofan ingot body by a distance which is 10 to 70 percent of the height ofthe ingot body for disrupting vertical heat ow in these walls, saidmeans including a thickened section and a thinner section im mediatelytherebelow, both of said sections` being of relatively short verticaldimensions. i

2. An ingot mold comprising solid side walls which define a cavityadapted to receive molten metal for casting an ingot body and whichextend a relatively short distance above the normal height of the ingotbody, said walls below this height being of greater thickness in theirlower portion than in their upper portion to accelerate solidiiicationof the lower portion of an ingot body both transversely and verticallyand to retard transverse solidification of the upper portion, the traceof the inside surfaces of said walls in any vertical plane away from thelower end being a straight line, a relatively short thick section insaid walls at the bottom of their upper thin portion spaced below thenormal height of an ingot body ,by a disseltlions cooperating to disruptvertical heat flow in said w s.

Y 3. A mold as defined in claim 2 which is ofthe big-endup type and inwhich the portion of the walls above the normal height of the ingot bodyis adapted to receive a hot top.

4. A mold as defined in claim 2 which is of the bigend down type and inwhich the portion of the walls above the normal height of the ingot bodyis adapted to receive a hot top when a hot-topped ingot is cast thereinand to provide clearance above the height of pour when a nonhot-toppedingot is cast therein.

References Cited in the le of this patent UNITED STATES PATENTS

1. AN INGOT MOLD COMPRISING SOLID SIDE WALLS WHICH DEFINE A CAVITYADAPTED TO RECEIVE MOLTEN METAL FOR CASING